STABILITY AND SELF-EXCHANGE IN ALKANETHIOL MONOLAYERS

Alkanethiols bearing radiolabeled (S-35) head groups have been incorpo rated into self-assembled monolayers (SAMs) on a variety of substrates to determine coverage, thermal- and photostability, and surface rough ness. Spontaneous desorption of octadecanethiol SAMs immersed in solve nt under ambient conditions is observed for all substrate/solvent comb inations, including gold, silver, platinum, and copper and water, etha nol, tetrahydrofuran and hexane. A quantitative analysis of desorption suggests pseudo-first-order kinetics with rate constants in the range of 10(-5) s(-1) for THF. Evidence for multilayering on copper is foun d. Molecular hydrogen is not involved in the rate-determining step for desorption. Self-exchange of surface-labeled thiol with solution unla beled thiol is also described by first-order kinetics. Both desorption and self-exchange experiments yield residual thiols at the surface wh ich cannot be exchanged, and which are presumed to result from stronge r binding at defect sites. Similarities in kinetics for exchange and d esorption point toward a common mechanism for surface detachment, post ulated to be a rate-limiting desorption step as a disulfide. Estimates for free energies of adsorption of a thiol generating molecular hydro gen and adsorption of a disulfide yield -5.5 and -24 kcal mol(-1), res pectively. In conjuction with recent evidence that thiols are actually adsorbed as disulfides, RSSRAu(2(s)), the desorption is represented b y RSSRAu(2(s)) --> RSSR + 2Au((s)) (slow). For exchange this is follow ed by 2RSH + 2Au((s)) --> RSSRAu(2(s)) + H-2 (fast), with some contrib ution from direct thiol/disulfide interchange for high concentrations of solution thiol, viz: RSSRAu(2(s)) + RSH --> RSSRAu(2(s)) + R*SH. O n clean gold surfaces, adsorption is shown to be diffusion limited. Fi nally, strategies for enhancing the stability of SAM monolayers made f rom thiols are discussed.